A major security alert has been sparked after police lost a briefcase full of imitation bombs.

An officer left the case containing around 15 mock-up explosive devices on a train.

He was returning home after using the "bombsî in a presentation to security staff and police.

Among the items are believed to be mock-ups of the device that caused the Lockerbie air disaster in 1988 and of those used by the Animal Liberation Front.

One source said: "They are very realistic. If the briefcase went through an x-ray machine you would see fuses and what looks like explosives.

"This could cause a massive security panic and chaos if one of these was found on the Tube or someone smuggled one onto a plane."

The incident is a major embarrassment to the City of London Police whose officer mislaid the case. The force's acting commissioner Mike Bowron ordered an immediate internal inquiry.

The dummy "bombs" are used in police presentations to alert fellow officers or corporate security staff to what they should be looking out for during searches for explosives.

The case is also believed to contain mock devices in the form of video cassettes.

The City officer who lost the briefcase immediately alerted British Transport Police after realising he had apparently left it on the train. BTP officers halted the train and conducted a search but could not find the case

Police had been hoping that it would be handed in after the incident on 31 October but it is still missing.

A City of London spokesman said: "We can confirm that a briefcase belonging to the City of London Police was lost or stolen on 31 October.

"The briefcase contained training materials used to educate the public about terrorism awareness.

"An internal inquiry is being carried out to ascertain the full circumstances around the loss of the material.

"We take incidents of this nature very seriously. We have also carried out an urgent review into how officers handle such material.

"A number of changes have already been made to ensure we minimise the risk of such incidents in the future."

Police hope that whoever has the case will return it intact, complete with dummy devices.

They say this person can return it to a police station or call Crimestoppers on 0800 555 111 with information on the briefcase's whereabouts.

In April, artist Monica Saieva sparked a bomb alert which paralysed west London when she left a series of suspicious boxes with nails sticking out of them in the street.

The bomb squad, forensic science officers and ambulances were called out before police realised the items were a hoax. The operation is believed to have cost £100,000. Ms Saieva was arrested and charged with causing a public nuisance but escaped with a caution.

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